COMPREHENSIVE TECHNICAL ANALYSIS AND DATASHEET OF ASTM A194 GRADE 4 FASTENING SYSTEMS
ASTM A194 Grade 4 specifies high-temperature, high-pressure heavy hex nuts manufactured from Carbon-Molybdenum alloy steel. Engineered as a direct metallurgical upgrade over standard carbon steel nuts (such as Grade 2H), Grade 4 introduces a highly specific addition of Molybdenum (0.20% – 0.30%). This critical alloying element vastly improves the material's high-temperature creep strength and structural stability.
To guarantee performance under severe thermal loads, Grade 4 nuts undergo strict quenching and tempering processes with a minimum tempering temperature of 1100°F (593°C). This ensures that the nut will not unexpectedly soften or relax its clamping force when exposed to the extreme operating temperatures of petroleum refineries, high-pressure boiler systems, and power generation turbines. They are frequently specified as the optimal mating hardware for ASTM A193 Grade B16 and Grade B7 high-strength bolting.
■ Product Overview & Specifications
Grade 4 nuts are supplied strictly in the quenched and tempered condition, providing robust mechanical strength, enhanced fatigue life, and dimensional stability under severe thermal and pressure cycling.
| Material Classification | Carbon-Molybdenum Alloy Steel (Nuts) |
|---|---|
| ASTM Specification | ASTM A194 / A194M Grade 4 |
| UNS Designation | K14032 (Approximate representation) |
| Magnetic State | Strongly Magnetic |
| Density | ~7.85 g/cm³ (0.283 lb/in³) |
| Temperature Limits | Up to +1000°F (+538°C) for continuous stress |
| Size Range | Metric: M6 to M100 | Imperial: 1/4" to 4" Standardly supplied as Heavy Hex Nuts for maximum thread engagement. |
| Thread Types | UNC, UNF, 8UN, Metric Coarse, Metric Fine |
■ Proprietary Datasheet Download (GATED)
Access professional-grade technical data for ASTM A194 Grade 4, including rigorous proof load testing charts, elevated-temperature strength retention, and long-term creep-rupture thresholds.
ASTM A194 Grade 4 — Complete Technical Datasheet
Contains precise heavy hex nut dimensional tolerances, high-temperature thread stripping calculations, and flanged joint torque maps for refinery and power generation operations.
⬇ DOWNLOAD DATASHEET■ MTC Integration Section
For pressure vessel compliance and extreme temperature safety, Ananka Group provides absolute metallurgical verification mapping back to the thermal processing batch.
- EN 10204 Type 3.1: Detailed reporting of actual chemical analysis (specifically verifying Molybdenum levels), mandatory hardness testing, and certified tempering temperatures (Min 1100°F).
- EN 10204 Type 3.2: Third-party witness testing facilitated for critical petroleum refinery piping and high-pressure steam boiler systems.
■ Chemical Composition
The chemistry of Grade 4 relies on a high carbon base for raw strength and hardenability, paired with a deliberate Molybdenum addition to prevent the steel from suffering microstructural creep (elongation) at high temperatures.
| Carbon (C) | Manganese (Mn) | Moly (Mo) | Silicon (Si) | Phosphorus (P) | Sulfur (S) |
|---|---|---|---|---|---|
| 0.40 – 0.50 | 0.50 – 0.95 | 0.20 – 0.30 | 0.15 – 0.35 | 0.040 Max | 0.050 Max |
*Unlike Grade 7 or Grade 3, Grade 4 does not contain significant Chromium. Its primary defense mechanism is mechanical strength at high heat rather than advanced corrosion resistance.
■ Mechanical Properties & Hardness Limits
ASTM A194 Grade 4 nuts undergo strict thermal processing to ensure they can support the extreme tension of high-strength alloy bolting without suffering thread shear or brittle fracture.
| Property | Grade 4 Requirements |
|---|---|
| Hardness (Maximum) | 352 HBW (~38 HRC) |
| Hardness (Minimum) | 248 HBW (~24 HRC) |
| Condition | Quenched & Tempered |
| Minimum Tempering Temp | 1100°F (593°C) |
| Proof Load / Stripping Strength | Designed to securely mate with extremely high yield strength alloy bolts (105-125 ksi yield). |
■ Equivalent Grades & Designations
| Standard / System | Designation |
|---|---|
| ASTM Specification | ASTM A194 Grade 4 (Heavy Hex Nuts) |
| Mating Bolt Spec (Primary) | ASTM A193 Grade B16 (Cr-Mo-V) |
| Mating Bolt Spec (Secondary) | ASTM A193 Grade B7 (Cr-Mo) |
■ Environmental & Mechanical Resistance Profile
Grade 4 is a workhorse alloy selected for demanding mechanical environments where ambient corrosion is less of a concern than extreme physical stress:
- Creep Resistance: The Molybdenum addition physically impedes the movement of dislocations within the steel's crystal lattice at high temperatures, preventing the nut from slowly stretching (creeping) under sustained load.
- Thermal Stability: The high tempering temperature (1100°F) guarantees that the nut will not undergo unexpected secondary tempering or softening when placed into service at temperatures up to 1000°F.
- Note on Corrosion: Grade 4 is a carbon-molybdenum steel, not stainless. It requires protective coatings (like PTFE, Zinc, or Cadmium plating) or a controlled, non-corrosive environment to prevent rusting.
■ Applications & Industries
ASTM A194 Grade 4 nuts are deployed heavily in high-temperature, non-corrosive processing environments:
- Power Generation: Fastening systems for high-pressure steam lines, boiler casings, and turbine housings.
- Petroleum Refining: Flange bolting for cracking units, hot crude piping, and high-temperature refinery heat exchangers.
- Heavy Manufacturing: High-stress press assemblies, die forging equipment, and heavy-duty industrial valves.
■ Compatible Fastener Assemblies
To ensure uniform thermal expansion and to prevent thread shear under massive clamping loads, Grade 4 nuts are engineered to mate with specific high-strength alloy bolts.
| Mating Nut (ASTM A194) | Recommended External Threads (Bolts/Studs) |
|---|---|
| A194 Grade 4 (Heavy Hex) | ASTM A193 Grade B16 (Chromium-Molybdenum-Vanadium Steel) |
| A194 Grade 4 (Heavy Hex) | ASTM A193 Grade B7 (Chromium-Molybdenum Steel) |
■ Torque & Installation Guidelines
Proper lubrication is essential to achieve accurate clamping force and to prevent thread galling under extreme tension. High-temperature anti-seize compounds (such as Copper or Molybdenum Disulfide based) are strongly recommended.
| Nominal Diameter | Threads Per Inch (UNC) | Target Torque (When mated w/ B16) — Lubricated (ft-lbs) |
|---|---|---|
| 1/2" | 13 | 60 - 80 |
| 5/8" | 11 | 120 - 150 |
| 3/4" | 10 | 200 - 250 |
| 1" | 8 | 480 - 550 |
■ Why Choose Ananka Group?
Ananka Group specializes in the complex manufacturing and documentation of high-temperature bolting for global pressure vessel and refinery applications.
- Heavy Hex Standards: All Grade 4 nuts are manufactured to Heavy Hex dimensions (ASME B18.2.2) to provide maximum thread engagement, preventing stripping under extreme high-temperature loads.
- Strict Thermal Processing: Tempering temperatures are rigorously monitored (1100°F minimum) to ensure the steel achieves the perfect balance of toughness and high-temperature stability.
- Custom Coatings: Because Grade 4 is susceptible to environmental corrosion, we offer a vast array of high-temperature resistant coatings, including Xylan/PTFE, Hot Dip Galvanizing, and specialized ceramic coatings.
- 100% Traceability: Every nut is permanently forged/stamped with "4" and the manufacturer's mark, backed by a comprehensive EN 10204 3.1 MTC.
■ Frequently Asked Questions
1. What is the difference between ASTM A194 Grade 2H and Grade 4?
Grade 2H is a quenched and tempered carbon steel nut suitable for high pressures but begins to lose creep strength rapidly above 800°F. Grade 4 is a Carbon-Molybdenum steel. The addition of molybdenum gives Grade 4 significantly better high-temperature creep strength, allowing it to operate safely at higher temperatures (up to ~1000°F) where Grade 2H would begin to slowly stretch and fail.
2. What is the difference between Grade 4 and Grade 7?
While both are high-temperature alloy steel nuts, Grade 4 is Carbon-Molybdenum (C-Mo), whereas Grade 7 is Chromium-Molybdenum (Cr-Mo). Grade 7 contains roughly 1% Chromium, which provides better oxidation resistance and slightly better hardenability compared to Grade 4.
3. Why pair Grade 4 with ASTM A193 Grade B16 bolts?
Grade B16 bolts contain Chromium, Molybdenum, and Vanadium for extreme high-temperature strength. Grade 4 nuts, with their Molybdenum content and high tempering temperature, possess the necessary mechanical strength and thermal stability to safely secure B16 bolts without becoming the weak point in the flanged joint.
4. Is Grade 4 suitable for NACE MR0175 sour service?
Grade 4 can be used in sour service environments, but its hardness must be strictly monitored and limited to a maximum of 22 HRC (237 HBW) to comply with NACE MR0175 / ISO 15156 guidelines for preventing sulfide stress cracking. Because standard Grade 4 nuts can be manufactured up to 352 HBW, NACE compliance must be explicitly requested at the time of order.